You can filter by category, or search by keyword (across the title and the description). One of the other neat features is that you can configure it to search only plugins compatible with a certain version of Confluence. You can limit your view to just plugins that will work with the version you’re running.
One thing to point out: there are several plugins in the Plugin Library that have not yet been put into the Plugin Repository. I’ve made a list of them. If you’re the author of one of these plugins, you should make sure to add them to the Repository, or else visitors won’t see them when searching and admins won’t see them in the product.